HYDROLYSIS OF CASSAVA STARCH BY MALTED ACHA (DIGITARIA EXILIS) FOR BIOETHANOL PRODUCTION
Energy and environmental issues are the major concerns facing the global community today (Hu et al., 2008). Renewable fuels (biofuels) such as bioethanol are becoming increasingly important due to heightened concern for the green house effect, depleting oil reserves and rising oil prices (Ohgren et al., 2007). Ethanol, chemically known as ethyl alcohol, is a clear, colourless liquid, with an agreeable odour (Bugaje, 2008). Bioethanol can be utilized as oxygenator of gasoline, elevating its oxygen content, allowing a best oxidation of hydrocarbons and reducing the amount of aromatic compounds and carbon monoxide released into the atmosphere (Cardona and Sanchez, 2007). Bioethanol is obtained from bioenergy crops and biomass which distinguishes it from that which is produced synthetically from petroleum (Ranola et al., 2009). Different countries use different bioenergy crops such as corn, cassava and sugarcane for bioethanol production. Cassava and sugarcane are used mainly in Nigeria and Brazil (Naylor et al., 2007). Experts have pointed out that cassava is the best energy crop for bioethanol production due to its comparative advantages over all known energy crops (Wang, 2002).
The ethanol yield of cassava per unit land area is the highest among all known energy crops (Wang, 2002).
High fermentable sugar content, stable shelf live, complete and easier hydrolysis, low cost of raw materials and simpler ethanol processing technology are the advantages of cassava flour over other flours (Grace, 1977; Ocloo, 2002 and Vijayagopal et al., 1980). Malted cereals have been used as sources of starch hydrolyzing enzymes, due to the fact that germination induces the synthesis of hydrolytic enzymes (Obatolu, 2002). These malted cereals are employed in the enzymatic saccharification of starch in most starch-based industries in Nigeria (Egwim and Oloyede, 2006). Barley and wheat malts give high yield of amylase and fermentable sugar but are quite expensive to import. However local sources can be used effectively for malt and enzyme production. Sorghum and acha (Digitaria exilis) have been shown to have higher germination capacity than other cereals such as maize and rice (Egwim and Oloyede, 2006). Sorghum alpha amylase have been shown to be the closest alternative to imported alpha amylase for industrial purposes and acha alpha amylase can also be as good as sorghum amylase and indeed could be a better source (Egwim and Oloyede, 2006). Development of hydrolytic enzymes was significantly higher in
Digitaria exilis than in sorghum (Nzelibe and Uwasike, 1995). Acha has been shown to has higher alpha amylase yield than sorghum, acha may be a better source of alpha amylase and can substitute for sorghum alpha amylase in industrial processing (Egwim and Oloyede, 2006). In the present study, malted acha (Digitaria exilis) was employed as local enzyme source in the hydrolysis of locally sourced cassava starch for bioethanol production.
Ethanol (CH3CH2OH) is a chemical compound which contains hydrogen, carbon and oxygen in its chemical structure. It is also known as ethyl alcohol or grain alcohol (U.S. EPA, 2008). It is a clear, colorless liquid with an agreeable odour (Bugaje, 2008). It is also referred to as the type of alcohol found in alcoholic beverages. Ethanol has a somewhat sweet flavor when diluted with water; a more pungent, burning taste when concentrated, it is more volatile than water, flammable, burns with a light blue flame, and has excellent fuel properties for spark ignition internal combustion engines (Wyman, 2004).
Figure 1: chemical structure of ethanol (Muhd, 2008)
Ethanol ranks second only to water as the most widely used solvent in chemical industry. It acts as solvent for an immense range of industrial products, including paints, lacquers, dyes and oils (Ocloo and Ayernor, 2010). It is used in medicine and motor fuels. It is also used in antifreeze compounds and rocket fuels, pharmaceuticals, printing and cosmetics.
HYDROLYSIS OF CASSAVA STARCH BY MALTED ACHA (DIGITARIA EXILIS) FOR BIOETHANOL PRODUCTION
1.2 Historical development of bioethanol as fuel
The use of ethanol as fuel dates back to 1826, when Samuel Morey developed an engine that ran on ethanol and turpentine called camphene. Bioethanol was used in Germany and France by the then incipient industry of internal combustion (IC) engines as early as 1894 (Demirbas and Karslioglu, 2007). Bioethanol as fuel gained more prominence in 1908, when the Ford Motors in the USA developed the Henry Ford‟s model T vehicle which was designed to use gasoline, ethanol (from corn) or a combination of both. The use of bioethanol for fuel was widespread in Europe and the United States during this period. Brazil has utilized bioethanol as transportation fuel since 1925. The potential of bioethanol was ignored, especially after the World War II, because it became more expensive than petroleum-based fuel. The energy crisis of the 1970‟s then renewed interest in ethanol production for fuel and chemicals in both the USA and Brazil, where mass production of bioethanol grown from corn and sugar cane started, respectively (Balat and Balat, 2009; Balat, 2009).
The United States is the world‟s largest producer of bioethanol fuel, accounting for nearly 47% of global bioethanol production. Brazil is the world‟s largest exporter of bioethanol and second largest producer after the United States (Balat and Balat, 2009). Brazil produces her bioethanol from sugarcane and cassava while the USA produces hers from corn (Naylor et al., 2007). China is also a leading contender in bioethanol production, producing over I billion litres per year from wheat and corn, while France which is leading other European countries, produces over 200 million gallons of ethanol from sugar beets and wheat (Sperling and Cannon, 2004).
Table 1: World bioethanol production during 2005 and 2006 (billion liters), (Balat and Balat, 2009).
Country 2005 2006 share total in 2006 (%)
USA 15.0 18.3 46.9
Brazil 15.0 17.5 44.9
China 1.0 1.0 2.6
India 0.3 0.3 0.8
France 0.15 0.25 0.6
Others 1.55 1.65 4.2
Total 33.0 39.0
Nigeria in 2004 joined the league of biofuel users, with a policy thrust according to the Nigeria National Petroleum Cooperation (NNPC), to generate fuel ethanol from cassava and sugar cane. This policy thrust was designed with the aim of generating wealth and reducing environmental pollution (Kupolokun, 2006; Umar, 2006). At present, the Federal Government of Nigeria has agreed to the blending of 5 percent ethanol (E5) by composition with premium motor spirit (PMS) (Ezeobi 2008). The government adopted E5 because it believes that the level will not damage vehicles in Nigeria, although this proportion is expected to increase to 10 per cent (E10) in the nearest future (Ugwuanyi, 2008). The core focus of the Nigeria biofuel programme is to ensure the production of fuel ethanol domestically.Bioethanol being a biofuel is produced from biological sources and has a lot of benefits which makes it a better energy source than fossil-based fuels.
1.3.1 Environmental benefit of bioethanol
Carbon dioxide emission due to combustion of fossil fuels has become a major environmental concern. Carbon dioxide emission contributes greatly to green house effect, climate change and global warming. Bioethanol, is primarily seen as a good fuel alternative because the source crops can be grown renewably and in most climates around the world. In addition, the use of bioethanol is generally CO2 neutral. This is achieved because, in the growing phase of the source crop, CO2 is absorbed by the plant and oxygen is released in the same volume that CO2 is produced in the combustion of the fuel. This creates an obvious advantage over fossil fuels, which emit CO2 as well as other poisonous emissions that have great negative impact on the environment (Cardona and Sanchez, 2007; Hu et al., 2008).
Bioethanol is bio-degradable, more environmentally friendly and less toxic than fossil fuel.
Also, bioconversion processes in general do not produce hazardous compounds, and if toxic solvents and chemicals are avoided in the processing stages, then fewer environmental pollutants are produced. In addition, biomass production and microbial conversion processes can be developed and used in a more distributed manner, avoiding the need for transport of fuels via cargo ships or pipeline for long distances (Drapcho et al., 2008). Also, CO2 from ethanol fermentation can be used to extract oils and nutraceutical compounds from biomass instead of using toxic organic solvents such as hexane (Walker et al., 1999). Bioethanol, can be used in biodiesel production from biological oils in place of toxic petroleum-based methanol traditionally used (Drapcho et al., 2008).
1.3.2 Respite to energy issues, depleting oil reserves and rising oil prices
Bioethanol has also become increasingly important as alternative energy source, due to depleting oil reserves and rising oil prices (Ohgren et al., 2007). Currently, available fossil fuel sources are estimated to become nearly depleted within the next century, with petroleum fuel reserves depleted within 40 years (Energy Information Agency, 2007). For instance, the United States imports 10 million barrels of oil per day of the existing world reserves (1.3 trillion barrels) as shown in table (1). Crude oil prices have risen from less than $20/barrel in the 1990‟s to nearly $100/barrel in 2007. The true cost of oil has been estimated as greater than $100/barrel since 2004 (Drapcho et al., 2008).
Bioethanol can create energy security and vary energy portfolio (Azmi et al., 2011). The U.S Energy Information Administration determined that total world energy consumption in 2005 was 488EJ (exajoule, 1018J). World consumption is expected to surpass 650EJ by 2025 (Energy information Agency, 2007).
Bioethanol, has emerged as the most suitable renewable alternative to fossil fuel, as their quality constituents match diesel and petrol respectively (Nugesha, 2009). Bioethanol is the most employed liquid biofuel either as fuel or as a gasoline enhancer. Table 2: World oil reserves and U.S import based on leading producers (Energy Information Agency, 2007).
Country Oil reserves (billion barrels) US oil imports (million barrels)
Saudi Arabia 267
United Arab Emirate
1.3.3 Socio-economic benefit of bioethanol
The utilization of biofuels has important economic and social effects. Sheeman and Himmel (1999) pointed out that the diversification of fuel portfolio would bring money and job back to the USA economy. Moreover, the development of energy crops dedicated to the biofuel production would imply a boost to agricultural sector. This analysis is also valid for developing countries (Cardona and Sanchez, 2007), of which Nigeria is one. Fuel ethanol production has increased remarkably, because many countries look for means of reducing oil import, boosting rural economies and improving air quality. Global biofuel demand is projected to grow 133% by 2020 (Kosmala, 2010) and this will be of great benefit to those economies involved in bioethanol production for export.
1.3.4 Advantages of bioethanol as engine fuel Adding bioethanol to gasoline increases the oxygen content of the fuel, which implies a less amount of required additive. The increased percentage of oxygen allows a better oxidation of the gasoline hydrocarbons with consequent reduction in the emission of CO and aromatic compounds (Malca and Freire, 2006). Wang et al (1999) corroborated this by writing that “using bioethanol blended fuel for automobiles can significantly reduce petroleum use and exhaust greenhouse gas emission”.
Bioethanol has a higher octane number, broader flammability limits, higher flame speed and higher heat of vaporization (Yoosin and Sorapipatana, 2007). These properties allow for higher compression ratio and shorter burn time, which lead to theoretical efficiency advantages over gasoline in an internal combustion engine (Balat, 2009). Octane number is a measure of the gasoline quality for prevention of early ignition, which leads to cylinder knocking. An oxygenated fuel such as bioethanol, with high octane number, provides a reasonable antiknock value (Balat and Balat, 2009). It is believed that a given volume of ethanol could provide energy enough to drive about 75-80% of the distance as the same amount of gasoline, although it has only about two-third of the energy content (Galbe and Zachi, 2002).
Bioethanol when related to MTBE (methyl tert butyl ether), which is also an oxygenator of gasoline, is not toxic and does not pollute ground water. Bioethanol is most commonly blended with gasoline in concentrations of 10% bioethanol to 90% gasoline, known as E10 and nicknamed “gasohol” (Oliveria et al., 2005). Bioethanol can be used as a 5% blend with petrol under the EU quality standard EN228. Bioethanol can be used at higher levels, for example, E85 (85% bioethanol) (Demirbas and Karslioglu, 2007)
1.3.5 Disadvantages of bioethanol as engine fuel
Bioethanol has lower energy density than gasoline (bioethanol has 66% of the energy that gasoline has). Also, the high oxygen content of ethanol and its ability to oxidize into acetic acid induce compatibility issues with some materials used in the engine, such as metals or polymers. In addition, ethanol leads to azeotropes with light hydrocarbon fractions and can lead to volatility issues, low flame luminosity, lower vapour pressure and high latent heat of vapourization (making cold starts difficult), miscibility with water (which can cause demixing issues when blended with hydrocarbons), toxicity to ecosystem (since its combustion in engines induces aldehyde emissions, which has negative impact on health) (Jeuland et al., 2004). Though the use of bioethanol as engine fuel has some disadvantages, its advantages as engine fuel far outweigh its disadvantages.
1.4 Feed stocks for bioethanol production
Bioethanol feedstock can be divided into three major groups: (1) starchy materials (2) sugar or sucrose-containing feed stocks and (3) lignocellulosic biomass